Functional cardiac imaging in mice using Ta-178

Assessment of right and left ventricular function in healthy mice by blood-pool pinhole gated SPECT

The feasibility of blood-pool pinhole ECG gated SPECT was investigated in healthy mice to assess right and left ventricular function analysis. Anaesthetized (isoflurane 1-1.5%) adult CD1 mice (n=11) were analyzed after intravenous administration of 0.2 ml of 550 MBq of (99m)Tc human albumin. For blood-pool gated SPECT imaging, 48 ventral step and shoot projections with eight time bins per RR over 180 degrees with 64 x 64 word images were acquired with a small animal gamma camera equipped with a pinhole collimator of 12 cm in focal length and 1.5 mm in diameter. For appropriate segmentation of right and left ventricular volumes, a 4D Fourier analysis was performed after reconstruction and reorientation of blood-pool images with a voxel size of 0.55 x 0.55 x 0.55 mm(3). Average right and left ejection fractions were respectively 52+/-4.7% and 65+/-5.2%. Right end diastolic and end systolic volumes were significantly higher compared with the corresponding left ventricular volumes (P<0.0001 each). A linear correlation between right and left stroke volumes (r=0.9, P<0.0001) was obtained and right and left cardiac outputs were not significantly different 14.2+/-1.9 and 14.1+/-2 ml/min, respectively.

Noninvasive cardiovascular phenotyping in mice

With the growth of genetic engineering, mice have become common as models of human diseases, which in turn has stimulated the development of techniques to monitor and image the murine cardiovascular system. Invasive methods are often more quantitative, but noninvasive methods are preferred when measurements must be repeated serially on living animals during development or in response to pharmacological or surgical interventions. Because of the small size and high heart rates in mice, high spatial and temporal resolutions are required to preserve signal fidelity. Monitoring of body temperature and the electrocardiogram is essential when animals must be anesthetized for a measurement or other procedure. Several other groups have developed cardiovascular imaging modalities suitable for murine applications, and ultrasound is the most widely used. Our group has developed and applied high-resolution Doppler probes and signal processing for measuring blood velocity in the heart and peripheral vessels of anesthetized mice noninvasively. We can measure cardiac filling and ejection velocities as indices of systolic and diastolic ventricular function and for timing of cardiac events; velocity pulse arrival times for determining pulse-wave velocity and arterial stiffness; peripheral velocity waveforms as indices of arterial resistance, compliance, and wave reflections; stenotic velocities for estimation of pressure drop and detection of vorticity; and tail artery velocity for determining systolic and diastolic blood pressure using a pressure cuff. These noninvasive methods are convenient and easy to apply and have been used to detect and evaluate numerous cardiovascular phenotypes in mutant mice.

Use of echocardiography for the phenotypic assessment of genetically altered mice

Transgenic mice displaying abnormalities in cardiac development and function represent a powerful new tool for understanding molecular mechanisms underlying normal cardiovascular function and the pathophysiological bases of human cardiovascular disease. Complete cardiac evaluation of phenotypic changes in mice requires the ability to noninvasively assess cardiovascular structure and function in a serial manner. However, the small mouse heart beating at rates in excess of 500 beats/min presents unique methodological challenges. Two-dimensional and Doppler echocardiography have been recently used as effective, noninvasive tools for murine imaging, because quality images of cardiac structures and valvular flows can be obtained with newer high-frequency transthoracic transducers. We will discuss the use of echocardiography for the assessment of 1) left ventricular (LV) chamber dimensions and wall thicknesses, 2) LV mass, 3) improved endocardial border delineation using contrast echocardiography, 4) LV contractility using ejection phase indices and load-independent indices, 5) vascular properties, and 6) LV diastolic performance. Evaluation of cardiovascular performance in closed chest mice is feasible in a variety of murine models using Doppler echocardiographic imaging.

Effects of different beta adrenoceptor ligands in mice with permanent occlusion of the left anterior descending coronary artery

1. We have studied the effects of three betaAR ligands (carvedilol, alprenolol, and ICI-118551) with different pharmacological profiles and negative efficacy at the beta2AR on cardiac in vivo, in vitro, biochemical and gene expression parameters in mice with permanent occlusion of the left anterior descending coronary artery. 2. Cardiac in vivo parameters were determined using Doppler studies. Mitral-wave E peak velocity (EPV) and aortic peak velocity (AoPV) decreased in the first 2 weeks postocclusion. After 3 weeks of drug treatment, EPV was improved in the carvedilol group to preocclusion values; however, a further reduction in EPV in the alprenolol and control permanent occlusion group was measured and there was no change after ICI-118551 treatment. AoPV was unchanged between weeks 2 and 5 in all groups. 3. The left atria were isolated to record isometric tension responses to isoprenaline. Permanent occlusion significantly reduced the maximum isoprenaline response to 30% of control and carvedilol increased the maximum response to isoprenaline significantly to 60%. 4. The biochemical and gene expression studies revealed different effects of the three betaAR ligands. Most notably, carvedilol reduced gene expression of myosin heavy chain beta. 5. These results indicate that chronic treatment with carvedilol is beneficial in a mouse model of myocardial damage resulting from ischaemia. We hypothesise that these beneficial effects of the drug may be because of the negative efficacy at the beta2AR, combined with beta1AR antagonism.

Heart failure and greater infarct expansion in middle-aged mice: a relevant model for postinfarction failure

Young mice tolerate myocardial loss after coronary artery ligation (CAL) without congestive heart failure (CHF) signs or mortality. We predicted a CHF phenotype after CAL in aged mice. Left coronary artery ligation produced permanent myocardial infarcts (MI). Mortality was higher in male 14-mo-old C57BL/6N mice (Older mice) than in 2-mo-old mice (Young mice) (16 of 25 Older mice died vs. 0 of 10 Young mice, P < 0.02). After 8 wk, rales, weight loss, and lethargy preceded deaths. Captopril (50 mg x kg(-1) x day(-1)) increased Older mouse survival (6 of 22 died, P < 0.02). Captopril improved systolic function (peak aortic blood velocity) from 76 +/- 6% of baseline in untreated Older mice to 93 +/- 8% (P < 0.036). At 24 h, MI comprised 28 +/- 4% of the left ventricle in Young mice, surprisingly larger than that in Older mice (18 +/- 2%, P < 0.011). Endocardial area underlying the infarct scar was significantly larger in Older mice than in Young mice. Captopril did not reduce expansion but markedly reduced septal hypertrophy. Aging reduces compensatory ability in mice despite smaller acute infarcts. Less effective myocardial repair, greater infarct expansion, and septal hypertrophy are seen with aging. Aging is a more relevant murine model of post-MI heart failure in patients.

Assessment of left ventricular systolic function using contrast two-dimensional echocardiography with a high-frequency transducer in the awake murine model of myocardial infarction

The estimation of global left ventricular function using M-mode echocardiography has technical limitations in the murine model of myocardial infarction (MI), but the recent improvements in 2-dimensional (2-D) echocardiography using a high-frequency transducer provide more accessible images. Furthermore, intravenous injection of contrast agent has the additional benefit of enhancing the endocardial border in the murine heart. The present study was designed to evaluate the value of 2-D echocardiography with intravenous injection of contrast agent in the assessment of global systolic function of the murine heart with MI. Two-dimensional and M-mode echocardiography without and with intravenous injection of contrast agent (Optison, 0.1-0.15 ml) were performed in 76 awake mice 2 days before and 2 days after left coronary artery ligation. Fractional shortening (FS) was calculated from the end-diastolic and end-systolic diameters on M-mode echocardiography, and fractional area change (FAC) from the end-diastolic and end-systolic areas on 2-D echocardiography. Both FS and FAC were compared with the areas of hypoperfusion observed in the pathological samples. The use of contrast agent improved the number of hearts that could be evaluated by both the M-mode and 2-D method (M-mode: non-contrast 87% vs contrast 99%, p<0.01; 2-D: non-contrast 26% vs contrast 89%, p<0.001). FAC from the 2-D method correlated better with the region of hypoperfusion in the pathological samples than did FS from the M-mode method (FAC: r=0.84 vs FS: r=0.51). In conclusion, FAC obtained from 2-D contrast echocardiography is useful for noninvasive assessment of global systolic function in infarcted murine hearts and can be used to serially assess systolic function in various models of the murine heart.

Decreased left ventricular ejection fraction in transgenic mice expressing mutant cardiac troponin T-Q(92), responsible for human hypertrophic cardiomyopathy

The causality of mutant sarcomeric proteins in hypertrophic cardiomyopathy (HCM) is well established. The current emphasis is to elucidate the pathogenesis of HCM in transgenic animal models. We determined the left ventricular ejection fraction (LVEF) in transgenic mice expressing mutant cardiac troponin T (cTnT)-Q(92), known to cause HCM in humans. Transgenes were constructed by placing wild-type (R(92)) or mutant (Q(92)) full-length human cTnT cDNAs 3' into a 5.5-kb murine [alpha -myosin heavy chain (MyHC)] promoter injected into fertilized zygotes. Three wild-type and six mutant lines were produced. Transgene mRNA and proteins, detected using transgene-specific probes were expressed at high levels in all wild-type and three mutant lines. The total cTnT mRNA pool was increased by up to five-fold in transgenic mice, but the total cTnT protein remained unchanged. The mean values of LVEF, determined by(178)Ta radionuclide angiography, were 57.8+/-6% (n=4) in non-transgenic littermate (NLM), 53.3+/-10 (n=6) in wild-type and 39. 4+/-6 (n=5) in mutant transgenic mice (P=0.009). The heart/body weight ratios and the number of cells stained with terminal deoxynucleotidyl transferase (TdT)-mediated nick end-labeling were similar among the groups. Three mutant mice had myocyte disarray and excess interstitial collagen and two had normal myocardial structure despite having reduced LVEF. Thus, in vivo expression of the mutant cTnT-Q(92)protein, responsible for human HCM, impaired global cardiac systolic function in transgenic mice, which also occurred in the absence of myocyte disarray and increased interstitial collagen.

Myocardial infarction and remodeling in mice: effect of reperfusion

Anatomic and functional changes after either a permanent left anterior descending coronary artery occlusion (PO) or 2 h of occlusion followed by reperfusion (OR) in C57BL/6 mice were examined and compared with those in sham-operated mice. Both interventions generated infarcts comprising 30% of the left ventricle (LV) measured at 24 h and equivalent suppression of LV ejection velocity and filling velocity measured by Doppler ultrasound at 1 wk. Serial follow-up revealed that the ventricular ejection velocity and filling velocity returned to the levels of the sham-operated controls in the OR group at 2 wk and remained there; in contrast, PO animals continued to display suppression of both systolic and diastolic function. In contrast, ejection fractions of PO and OR animals were depressed equivalently (50% from sham-operated controls). Anatomic reconstruction of serial cross sections revealed that the percentage of the LV endocardial area overlying the ventricular scar (expansion ratio) was significantly larger in the PO group vs. the OR group (18 +/- 1.7% vs. 12 +/- 0.9%, P < 0.05). The septum that was never involved in the infarction had a significantly (P < 0.002) increased mass in PO animals (22.5 +/- 1.08 mg) vs. OR (17.8 +/- 1.10 mg) or sham control (14.8 +/- 0.99 mg) animals. Regression analysis demonstrated that the extent of septal hypertrophy correlated with LV expansion ratio. Thus late reperfusion appears to reduce the degree of infarct expansion even under circumstances in which it no longer can alter infarct size. We suggest that reperfusion promoted more effective ventricular repair, less infarct expansion, and significant recovery or preservation of ventricular function.